In recent years, in regard to ultrahigh-speed communications exceeding 100 Gb/second, studies have been vigorously conducted on digital coherent communication by Dual Polarization Differential Quadrature Phase Shift Keying (DP-QPSK) excelling in wavelength use efficiency, receiving characteristics, and dispersion compensation capability. A receiver for DP-QPSK system requires a function to split an optical signal into polarized waves, and a 90-degree optical hybrid function for retrieving phase information from the split optical signals. The standardization of receivers having such functions has been studied by OIF (Optical Internetworking Forum), an industry group promoting high-speed data communications, and the development of receivers that comply with the specifications for the standardization has been vigorously pursued.
Incidentally, it is said that a planar lightwave circuit using optical waveguide technology is influential as a function to achieve the receiver of such DP-QPSK system. A silica waveguide is used for the planar lightwave circuit. However, it is difficult for the silica waveguide to increase a relative index difference between a core and a clad. The relative index difference of the silica waveguide is generally about 2%. Accordingly, the minimum bend radius of the waveguide is of the order of mm, and thus it is difficult to miniaturize a chip. Under such circumstances, recent interest has been focused not only on a coherent mixer, but also on a silicon waveguide (Patent Literature 1). Since silicon has a high refractive index of about 3.5, an excess loss is not generated even if the silicon waveguide is bent with a radius in the order of several μm to several hundreds of μm.